The invention relates generally to the field of digital image processing and, more particularly, to a method for preserving image detail when adjusting the tone scale of a digital color image.
Existing methods for applying tone scales to digital color images include applying the tone scale to each of the color channels independently, applying the tone scale to the neutral channel and preserving the original pixel color difference signal, or applying the tone scale to just the low frequency image. These methods of tone scale application alter the sharpness and appearance of the color in processed images, and the latter can produce displeasing artifacts.
Traditional methods of applying a tone scale function to a digital color image modify the apparent sharpness of the image because the tone scale modifies the amplitudes of high frequency detail information. This phenomenon occurs when the tone scale function is applied to each of the color channels independently, or when the tone scale function is applied to the neutral channel and original pixel color difference signals are preserved.
In an effort to apply a tone scale function to a digital image without distorting the detail information, in U.S. Pat. No. 5,012,333, Lee et al. proposed separating the image into a high frequency and a low frequency image by using FIR filters. The tone scale function is then applied to only the low frequency image, and the high frequency image is added back to the tone scaled low frequency image.
Also, in U.S. Pat. No. 5,454,044, Nakajima suggests modifying the image contrast by the formula
Sproc=Sorg+f(Sus).
The low frequency image (Sus) is passed through function f( ) which is a monotonically decreasing function. This signal is added to the original (Sorg) to create the processed image Sproc.
Both of these methods preserve the high frequencies of the image, but this may lead to the creation of unsharp mask type artifacts (overshoot and undershoot) in the neighborhood of large edges (characteristic of occlusion boundaries or dark shadows).
Consequently, a need exists for overcoming the above-described drawbacks. More specifically, a need exists for permitting the application of a tone scale function to a digital image to adjust the macro contrast of the image, preserves the high frequency detail information, and prevents artifacts in the neighborhood of large edges.
It is the object of the present invention to provide a more effective means of applying a tone scale function to a digital color image with the result that the processed image retains most of the spatial detail of the original. According to one aspect of the invention, the invention resides in a method for preserving detail when adjusting tone scale of a digital color image, the method comprising: receiving a digital color image having at least two color channels; providing a tone scale function for adjusting the tone scale of the digital color image; generating a luminance signal from the digital color image; generating a control signal derived from the luminance signal; using the tone scale function, and the control signal applied to each color channel of the digital color image to produce an enhanced digital color image.
The present invention has the advantage of preserving spatial detail and color edge detail while altering the tone scale of a digital color image.